A gene argH, encoding argininosuccinate lyase (ASL), has been cloned from a cosmid library of the filamentous cyanobacterium Nostoc sp. strain PCC 73102. The argH open reading frame encodes a protein comprised of 461 amino acids with a calculated molecular mass of 51,349 Da. Protein sequence comparisons reveal significant similarities of the Nostoc PCC 73102 ASL to related proteins from other organisms. In an Escherichia coli Delta argH strain, the Nostoc PCC 73102 ASL expressed from a recombinant plasmid could restore the ability to grow on medium without arginine. Moreover, cell extracts show a specific ASL activity of 16.2 nmoles of urea.min(-1).(mg protein)(-1). Partially purified, His-tagged ASL runs as a 53-kDa protein band in SDS-PAGE and about 215-kDa protein in native-PAGE, suggesting that the native protein is a tetramer.